The present invention relates to a power supply system for electric discharge machining ("EDM") or other electroerosion machining equipment, and more particularly to an EDM power supply having an improved response to short-circuit conditions at the machining gap.
In conventional EDM equipment, a current limiting resistance is positioned in series with the gap to prevent excess current through the gap. However, when the gap experiences a short-circuit condition, a current which is greater than the normal machining current passes through the gap, usually causing an arcing condition.
In order to prevent arcing, a chopper circuit, i.e. a circuit in which current is intermittently passed through the gap during a single discharge period to lower the average power, has been used. An example of such a conventional chopper circuit is shown in FIG. 1. FIGS. 2(B) through 2(D), are timing charts which illustrate the gap voltage and current behavior in the circuit of FIG. 1 in response to the gate signal of FIG. 2(A). In FIG. 1, a current sensing resistance 251, a coil 252, a switching transistor 253 and the machining gap 240, are connected in series with a power supply 250. A current sensor 254 detects the current passing through the gap 240 via the current sensing resistance 251 and outputs a gate 1 signal which, when it is high, i.e., a digital 1, switches the transistor 253 to an ON position. The time during which the signal from the sensor 254 is high, determines the so-called ON period of the machining pulse or the period during which machining power is applied to the gap.
In the chopper circuit of FIG. 1, a coil 252 is used in order to produce a smooth machining current. However, the coil 252 acts to delay the start of discharge current flow through the gap; thus the integrated value of the discharge current through the gap is reduced and the EDM machining rate is reduced. As is known to one of skill in the art, as the discharge current pulse width is shortened, the machining rate is reduced. The gap current, for large and small values of coil 252 inductance (L), is illustrated in FIGS. 2(C) and 2(D), respectively.
As shown in Japanese Patent No. 62-27928, a signal having a considerably higher frequency than that of the gate signal (as shown in FIG. 2(A), the gate signal is a signal which stays high or at an ON-position during a period corresponding to the time during which voltage is applied to the machining gap) is supplied to a switching transistor, such as the transistor 253 in FIG. 1. When the current waveform is smoothed by an inductance section, such as the inductance 252 of FIG. 1, a relatively constant current is supplied to the gap. This generally constant current type of circuit is well known and, since the difference between the short-circuit current and a machining current is small, can usually prevent the induction of a short-circuit arc. If the power supply voltage is low, based on the time constant established by the current sensing resistance 251 and the inductance of the coil 252, steep current increases are prevented, thus resulting in the above-noted problem in that the machining rate is reduced.
In the example disclosed in Japanese Patent No. 62-27928, the magnitude of each of the currents in the discharge current waveform is depressed due to the effect of circuit inductance. In particular, when the peak machining current is 10 amperes or less, the machining rate becomes extremely low.
In order to reduce the energy required, attempts have been made to pass the machining current through the machining gap while the voltage level is reduced. Thus, the no-load voltage is reduced, and the machining waiting time, [.tau.w], is extended. The waiting time is the time between the instant when voltage is applied to the machining gap and the time when a discharge current starts to flow. It is best illustrated in the left-hand part of the gap voltage depiction in FIG. 2(B). As a result of longer waiting times, the machining rate is reduced.
In EDMs which use water as a dielectric, an electrolytic current passes through the gap during the waiting time so that the gap voltage is reduced, sometimes causing a problem in that discharge does not occur.
Further, it is conventionally recognized that when selecting machining conditions for EDMing, the machining rate is reduced when electrode consumption is limited, but that generally electrode consumption is high when the machining rate is high. Therefore, the user must decide whether to emphasize electrode consumption or the machining rate in setting machining conditions. In the above-mentioned prior art example, there is a problem in that the discharge conditions for achieving minimal electrode consumption and high machining rate cannot be achieved.
In conventional systems, the generation of a step-like machining current waveform required use of a large number of transistors to pass the current. The entire system became large and complicated.
Further, as shown in FIG. 2(D), when a short-circuit condition exists at the gap, in conventional power supplies the waveform of the discharge current takes on an exaggerated zigzag shape, resulting in unexpected machining processes. For example, electrode consumption becomes extremely high, and the working rate is extremely low.